Detection of HC$_5$N and HC$_7$N Isotopologues in TMC-1 with the Green Bank Telescope

TL;DR

This study reports the first interstellar detection of specific isotopologues of cyanopolyynes in TMC-1, providing insights into their formation chemistry and isotopic ratios using Green Bank Telescope observations.

Contribution

It presents the first detection of DC$_7$N and six $^{13}$C isotopologues of HC$_7$N in space, and discusses their formation pathways and isotopic compositions.

Findings

Detected DC$_7$N and six $^{13}$C isotopologues of HC$_7$N in TMC-1.

Isotopic ratios are consistent with previous measurements, with no significant $^{13}$C variation in HC$_7$N.

HC$_5$N appears isotopically depleted, suggesting formation from smaller cyanopolyynes or hydrocarbon ions.

Abstract

We report the first interstellar detection of DC$_7$N and six $^{13}$C-bearing isotopologues of HC$_7$N toward the dark cloud TMC-1 through observations with the Green Bank Telescope, and confirm the recent detection of HC$_5$$^{15}$N. For the average of the $^{13}$C isotopomers, DC$_7$N, and HC$_5$$^{15}$N, we derive column densities of 1.9(2)$\times$10$^{11}$, 2.5(9)$\times$10$^{11}$, and 1.5(4)$\times$10$^{11}$ cm$^{-2}$, respectively. The resulting isotopic ratios are consistent with previous values derived from similar species in the source, and we discuss the implications for the formation chemistry of the observed cyanopolyynes. Within our uncertainties, no significant $^{13}$C isotopomer variation is found for HC$_7$N, limiting the significance CN could have in its production. The results further show that, for all observed isotopes, HC$_5$N may be isotopically depleted relative to HC$_3$N and HC$_7$N, suggesting that reactions starting from smaller cyanopolyynes may not be efficient to form HC$_{n}$N. This leads to the conclusion that the dominant production route may be the reaction between hydrocarbon ions and nitrogen atoms.